Mediterranean diet link to stronger bones

"Just two years of eating like the Spanish and Italians who use olive oil rather than less healthy fats may preserve or even build bone in older people," the Daily Mail reports.

The story is based on a study of whether the so-called Mediterranean diet – a diet rich in fresh fruit, vegetables, fish and olive oil – could strengthen bones and protect against osteoporosis.

The researchers were comparing the long-term effects of three different types of diet on blood levels of osteocalcin. Osteocalcin is a protein involved in the development of new bone.

In the study, three types of diet were assigned to older men, with an average age of 68. The three diets were:

a Mediterranean diet enriched with virgin olive oil

a Mediterranean diet enriched with nuts

a low fat control diet

Researchers found that in men assigned to a Mediterranean diet enriched with virgin olive oil, levels of osteocalcin and other markers for healthier bone formation were significantly higher at the end of the study’s two-year period than they were at the start. In men on the other two diets levels of these markers were not significantly different after two years.

The results of this study are of interest, especially in the light of earlier research suggesting that southern Mediterranean countries have lower levels of osteoporosis than countries in northern Europe. It may well be the case that high consumption of olive oil is associated with increased production of osteocalcin, which has a protective effect on bone. However, this is still a theory and not proven by this study.

To investigate this issue further, research that measures the effects of diet on bone strength (density) directly is required.

Where did the story come from?

The study was carried out by researchers from various institutions in Spain. The lead author is based at the Hospital Dr Josep Trueta in Girona. It was partly funded by grants from the Spanish government. The study was published in the peer-reviewed Journal of Clinical Endocrinology and Metabolism.

Its findings were overstated by the Mail’s claim that swapping to a Mediterranean diet rich in olive oil “for just two years” could help protect bones in later life. While the study did find that men in the olive oil group had higher levels of the osteocalcin protein, there is, as yet, no conclusive proof that this will lead to protection against osteoporosis.

What kind of research was this?

This was a randomised controlled trial (RCT) comparing the effects of three different types of diets on blood levels of a marker called osteocalcin, as well as other compounds, in a group of older men.

The three diets under investigation were:

a Mediterranean diet enriched with virgin olive oil

a Mediterranean diet enriched with nuts

a low fat control diet

The authors point out that nutritional factors are known to be involved in age-related bone loss and that studies have shown the incidence of osteoporosis to be lower in the Mediterranean basin.

Animal studies have suggested, they say, that consumption of olives and olive oil can prevent loss of bone mass. Laboratory studies have also indicated that oleuropein, a constituent of virgin olive oil, may have a protective effect, as well as being involved in the regulation of glucose in the blood.

However, there have been few, if any, studies looking at the effects of olive oil on blood levels of markers associated with formation of new bone in humans.

What did the research involve?

Participants in the study were 127 men, aged 55 to 80, who were randomly selected from a larger trial looking at the possible role of the Mediterranean diet in the prevention of cardiovascular disease.

The men had either been diagnosed with type 2 diabetes or they had at least three risk factors for cardiovascular disease, such as:

high blood pressure

high cholesterol levels

being overweight or obese

a family history of premature cardiovascular disease

There were several exclusion criteria for the trial including the use of medications known to affect bone formation or calcium levels.

The men were randomly assigned to three groups:

one group was advised to use the Mediterranean diet supplemented with virgin olive oil

one group was advised to use the Mediterranean diet supplemented with mixed nuts

a control group was advised to go on a low fat diet

All participants were given the same personalised dietary advice by a dietitian. The dietitian recommended:

the use of olive oil in cooking and dressing food

increased consumption of fruit, veg, pulses and fish

the replacement of red and processed meat with white meat

the avoidance of butter and cream

In addition

Those on the low fat diet were advised to reduce all types of fat from both animal and vegetable sources.

Those on the Mediterranean diet with olive oil were advised to replace refined olive oil or other vegetable oils with extra virgin olive oil, which the authors say retains certain natural phytochemicals and antioxidant compounds. The advice was to consume at least 50ml of virgin olive oil daily.

Those on the Mediterranean diet with nuts were allotted 30g of walnuts, almonds and hazelnuts daily.

Participants were given a short questionnaire about lifestyles, medical conditions and medication use at the start of the study and at one and two years’ follow-up. A validated food questionnaire was also used yearly to assess their dietary intake and a questionnaire was used to measure physical activity.

At the start of the study and after two years, researchers took blood samples from the men and measured levels of osteocalcin and P1NP, which are both markers for the formation of new bone. They also measured blood levels of a marker known as CTX, which is involved in the resorption of old bone. Bone resorption is a process in which older bone cells are broken down in order to release calcium to make new bone cells.

Other markers were used to measure the men’s insulin resistance, which is an indication of their risk of diabetes.

What were the basic results?

Researchers report that at the start of the study, the markers for bone formation were similar in all groups, as were other characteristics such as age, BMI and cholesterol levels. But after two years:

the total osteocalcin levels had increased significantly in the group on a Mediterranean diet with olive oil but not in the other two groups

P1NP also increased significantly in the group on a Mediterranean diet with olive oil but not in the other two groups

calcium levels decreased significantly in those on a Mediterranean diet with nuts and the low fat group but not in those on the Mediterranean diet with olive oil

overall, consumption of olives had a positive association with osteocalcin levels both at the start of the study and after two years

the marker for bone resorption, CTX, decreased significantly in all study groups – which presumably was due to the natural effects of ageing on bone strength

How did the researchers interpret the results?

The researchers say that consumption of a Mediterranean diet enriched with virgin olive oil for two years is associated with increased levels of two markers for bone formation, which suggests this diet has a protective effect on bone.

Conclusion

This was a well conducted study and the results are of interest. Its strengths include its randomisation of participants to different groups and its relatively lengthy follow-up period. However, the study also has limitations.

The most important of these is that it did not measure the men’s bone density or fracture rate, only certain “surrogate” or intermediate blood markers associated with bone turnover, so it cannot show that a particular type of diet protects against bone loss.

In addition, the participants were originally recruited for the PREDIMED study, to look at the impact of diet on risk of diabetes or heart disease.

People recruited to the PREDIMED study were older men with pre-existing risk factors for diabetes or heart disease. So the results of this study may not apply to other populations, such as younger people or women.

It is also possible that the men did not always stick to the dietary advice given, nor accurately recall their dietary intakes over each year, which may affect the reliability of the study results.

The Mediterranean diet has proven health benefits but, as yet, we do not know if it protects against osteoporosis, a complex condition involving not only diet but also genetic susceptibility, hormones and other lifestyle factors such as physical activity. To find out if the Mediterranean diet, with or without virgin olive oil, protects against bone loss, a long-term randomised controlled trial recruiting a wider age range of men and women for this sole purpose is required. It should measure not only bone density but also the incidence of fracture.